This invention relates to blowing articles from a thermoplastic material and more particularly to an improved shell mold and method for thermal conditioning a thermoplastic parison prior to blow molding the parison into a hollow finished article.
Bottles and other hollow articles are sometimes blown from thermoplastic materials such as polypropylene, polyethylene, polyethylene terephthalate and polyvinyl chloride. As an initial step in manufacturing a bottle, a parison is formed either by free extruding a mass of the thermoplastic material into a hollow tube and subsequently blowing the extrusion in a preform mold, or by injection molding. When the parison is extruded, variations in wall thickness may be achieved by programming the extrusion nozzle to provide additional glass at locations which are stretched the most during the final molding steps. The parison is then transferred to a blow mold cavity where it is expanded by blowing into contact with the mold. By cooling the parison to a uniform temperature between the crystalline freezing point and the melting point of the material prior to blowing, the material becomes biaxially oriented as it is stretched during blowing. The degree of orientation increases as the temperature of the material during stretching approaches an optimum value. As a consequence, it is desirable to maintain the parison at a uniform temperature close to the optimum value to achieve uniformly high biaxial orientation in the blown bottle or other container. Biaxial orientation requires at least some stretching in two directions. When a parison is blown or inflated into a bottle, the sides are stretched in two directions. Orientation in the side walls of the bottle can be increased by stretching the parison by extending a plunger through a preformed bottle neck after the parison is inserted into the final blow mold, either prior to or simultaneously with blowing.
It has been recognized that the temperature history of the parison must be carefully controlled prior to blowing in the final blow mold to achieve optimum orientation and/or uniform wall thickness. The time of retention of the thermoplastic material in the preform mold and the time of transfer of the preformed parison to the final blow mold for blowing are all carefully correlated with the wall thickness of the preformed parison and of the final article and with the material utilized in the operation, so that the final blowing is accomplished under conditions most conductive to orientation. The thermal history of the parison prior to blowing is also critical for controlling the final wall thickness in the finished article. For materials such as polyvinyl chloride, the final thickness of the oriented material is extremely sensitive to thermal variations prior to stretching. For typical blow molding stretch ratios, a thickness variation of 50% or more can be expected to result from a 5.degree. F. temperature non-uniformity. The thickness variations of the final bottle resulting from the thermal non-uniformities in the preformed parison become progressively worse as the degree of stretch by blow out or plunger stretch is increased with other factors or functions remaining unchanged.
The prior art has suggested zone cooling as a method for thermal conditioning a preform. In the case of an extruded preform, the zone cooling takes place in the preform mold where the parison is held for a short time after blowing. For an injection molded parison, the zone cooling takes place either in the injection mold or in a preform mold, or both. However, zone cooling has required a complicated mold with a large number of fluid connections for circulating different temperature cooling fluids through different passages formed in the mold. For a two piece mold having four different cooling zones, sixteen separate fluid connections are required since a separate fluid inlet and outlet must be provided for each zone in each mold half. A problem with prior art zone cooling techniques occurs when it is necessary to change the location of any cooling zone. This has required the construction of a new mold in most cases, resulting in a high expense. This is of particular importance because the optimum locations for the different zones are determined primarily through trial and error techniques.